Oil and gas exploration and production are very expensive operations. Any knowledge about the formations that can help reduce the unnecessary waste of resources in well drilling will be invaluable. Therefore, the oil and gas industry have developed various tools capable of determining and predicting earth formation properties. Among different types of tools, nuclear magnetic resonance (NMR) instruments have proven to be invaluable. NMR instruments can be used to determine formation properties, such as the fractional volume of pore space and the fractional volume of mobile fluid filling the pore space.
Nuclear magnetic resonance is a phenomenon occurring in a selected group of nuclei having magnetic nuclear moments, i.e., non-zero spin quantum numbers. Common nuclei with magnetic moments (xe2x80x9cnuclear spinsxe2x80x9d) include 1H (proton), 13C (carbon-13), 19F (fluorine-19) and 31P (phosporus-31). Hereafter, xe2x80x9cnuclear spinsxe2x80x9d will be used to refer to magnetic moments of nuclei. When these nuclei are placed in a magnetic field (Bo, xe2x80x9cZeeman fieldxe2x80x9d), they each precess around the axis of the Bo field with a specific frequency, the Larmor frequency (xcfx89o), which is a characteristic property of each nuclear species (gyromagnetic ratio, xcex3) and depends on the magnetic field strength (Bo) effective at the location of the nucleus, i.e., xcfx89o=xcex3Bo.
A typical NMR tool comprises a device that is moveable in a well bore. The device comprises a permanent magnet, which is used to induce a static magnetic field that aligns the nuclei of interest along the axis of the magnetic field (which is conventionally referred to as the Z-axis), and an antenna, which is used to provide radio frequency (RF) pulses and to act as a receiver for the resulting resonance signals. The RF pulses transmitted through the antenna produce a magnetic field (B1 magnetic field) which realigns the nuclei in a different orientation. Conventionally, the RF magnetic field is applied in the X or xe2x88x92X axis direction. This results in the net magnetization of the nuclei (nuclear spin) to mutate towards the xe2x88x92Y or Y axis, respectively. The X and Y axes refer to the axes in the rotating frame that is customary in the art. In a conventional spectrometer, the receiver is designed to measure the Y-axis and/or X-axis component of the magnetization as the latter precesses around the Z axis.
In a typical application, the RF pulse strength is controlled such that the nuclear spins are realigned onto a plane which is perpendicular to the direction of the magnetic field generated by the permanent magnet. Such RF pulse is called a 90-degree pulse because it nutates nuclear spins by 90 degrees (from the Z-axis to the Y-axis). Similarly, a pulse that nutates the nuclear spins from the Z-axis to the Z axis direction is referred to as a 180-degree pulse. Once in this perpendicular plane, the interactions between the static magnetic field and the nuclei cause these nuclei to precess around the static magnetic field axis with a characteristic frequency called Larmor frequency. The precessing of these nuclei produces signals that are detected by the antenna. In the absence of further perturbation, these nuclei will gradually return to their steady states, in which their net spin moments are aligned with the static magnetic field. The process of this return to the steady state is referred to as the spin-lattice (longitudinal) relaxation and is defined by a life time called T1. A separate process, spin-spin (transverse) relaxation, is also available by which the nuclear spins lose their detectable magnitudes. The spin-spin relaxation is defined by a lifetime T2, which is typically less than or equal to T1. T2 relaxation is typically investigated with pulse sequences that permit acquisition of NMR data that are more suitable for T2 relaxation analysis, e.g., a Carr-Purcell-Meiboom-Gill (CPMG) sequence. The signal magnitudes measured by a CPMG sequence decay exponentially by the spin-spin relaxation mechanism. The T1 and T2 values reflect the chemical and physical properties of the observed nuclei. Therefore, they can provide information as to the properties and the environment of the nuclei.
Most NMR tools used in earth formation analysis measure the spin-lattice relaxation times (T1) or spin-spin relaxation times (T2) to derive the properties of the earth formations. T2 relaxation is often measured from a train of spin-echoes that are generated with a series of pulses such as the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence or some variant of this. The CPMG pulse sequence is well known in the art. See Meiboom, S., Gill, D., 1958, xe2x80x9cModified Spin Echo Method for Measuring Nuclear Relaxation Times,xe2x80x9d Review of Scientific Instruments, 29, 688-91.
A CPMG sequence typically comprises a 90-degree (excitation) pulse followed by a series of 180-degree pulses (refocusing pulses or inversion pulses) with a fixed delay time between them. The delay times between the 180-degree pulses are roughly twice as long as that between the 90-degree and the first 180-degree pulses. The initial 90-degree pulse aligns the nuclear spins in the plane perpendicular to the direction of the magnetic field induced by the permanent magnet. The successive 180-degree pulses keep these nuclear spins roughly in this plane for the duration of the measurement The nuclear spins in the transverse plane decay mainly via the spin-spin relaxation (T2) pathway. Thus, one can derive the T2 relaxation time by analyzing the exponential decay of the spin-echo magnitudes.
NMR logging commonly detects 1H (proton) signals because proton is among the most abundant and easily detectable nuclei. These measurements do not include information on the couplings (scalar couplings or J-couplings) between the observed protons and other hetero nuclei because a typical CPMG sequence does not permit acquisition of such hetero coupling information. Scalar couplings or J couplings arise from through bond interactions, in which two nuclei connected by covalent bonds influence each other. Assuming a nucleus A under investigation has a neighbor B located one to three covalent bonds away (e.g., the CH3 and CH protons in acetaldehyde, CH3xe2x80x94CHxe2x95x90O), nucleus A would have its neighbor B in the low-spin state half of the times and in the high-spin state the other half of the times. As a result, the NMR signal of nucleus A would appear as a doublet, separated by a coupling constant J Hz. The magnitude of the J coupling constant depends on the types of nuclei involved and the separation (how many covalent bonds) between the nuclei. If the coupling is between two different types of nuclei (e.g., 1H and 13C), it is referred to as heteronuclear coupling. If it is between the same type of nuclei (e.g., 1H and 1H), the coupling is homonuclear coupling. Most J couplings are detectable if the coupled nuclei are separated by three or fewer covalent bonds. The closer the coupled nuclei are to each other, the stronger the J couplings are.
The scalar (J) couplings, because they depend on the types of nuclei involved and the separation between the coupled nuclei (and some times the geometry of the molecules), may provide information regarding the structures of the molecules under investigation. For example, the hetero J couplings between C and H are about 125-130 Hz for aliphatic compounds and about 150 Hz for aromatic compounds. This molecular information may be invaluable in characterizing formation fluids. Therefore, it is desirable to have apparatus and methods for formation fluid analysis that can provide scalar coupling information.
One aspect of the invention relates to NMR instruments for well logging or formation fluid sampling and analysis. These NMR instruments are capable of measuring NMR data that include heteronuclear or homonuclear coupling modulations. A nuclear magnetic resonance instrument according to embodiments of the invention includes a housing adapted to move in a wellbore; a magnet disposed in the housing adapted to induce a static magnetic field having a selected magnetic field strength in a zone of interest; an antenna assembly disposed in the housing, the antenna assembly adapted to resonate at a first frequency and a second frequency, the first frequency corresponding to a resonance frequency of a first nucleus at the selected magnetic field strength, the second frequency corresponding to a resonance frequency of a second nucleus at the selected magnetic field strength, wherein the first nucleus is different from the second nucleus; means for inducing a radio frequency magnetic field according to a selected pulse sequence in the zone of interest, the means for inducing the radio frequency magnetic field being operatively coupled to the antenna assembly; and means for detecting nuclear magnetic resonance signals at the first frequency, the means for detecting being operatively coupled to the antenna assembly.
Another aspect of the invention relates to methods for determining a formation property with a nuclear magnetic resonance instrument in a wellbore using J-spectroscopy. A method according to the invention includes inducing a static magnetic field having a selected magnetic field strength in an earth formation sample; acquiring nuclear magnetic resonance measurements having J coupling information using the nuclear magnetic resonance instrument; and deriving the J coupling information from the nuclear magnetic resonance measurements. The acquiring NMR measurements may be performed with a pulse sequence comprising a CPMG pulse sequence.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.